feat(sync): use page index as canonical locator for fixed-layout & comic formats

Fixed-layout EPUBs, PDFs, DjVu, and comic archives (cbz/cbr/cb7/cbt)
are page-based: each page is a fixed image, so a page index is an exact,
universal locator regardless of screen size or device. Sync previously
treated these like reflowable content (CFI-first restore, percentage
fallback, character-offset math), which was both wrong and lossy. This
makes the page index the canonical position for fixed-layout and comic
formats while leaving the reflowable path byte-for-byte unchanged.

Backend:
- progress.go SaveProgress: branch on mediaItem.FormatGroup. For
  fixed_layout/comic_archive derive percentage from current_page/
  total_pages and skip the CFI/character-offset back-fills (meaningless
  for image content). The reflowable derivation block is preserved
  verbatim under an else.
- kobo.go: Kobo only sends a percentage, so for fixed-layout/comic
  formats derive CurrentPage via PercentageToPage(percentage, pageCount)
  using the media item's known page count, so Kobo->web lands on the
  exact page.

Reader:
- reader.templ readerInitExpr: pass savedPage/savedTotalPages to the
  reader config for fixed_layout/comic_archive formats.
- reader.ts: when isFixedLayout and savedPage is present, restore via
  view.init({ lastLocation: savedPage - 1 }) (a bare number navigates
  foliate directly to the section index). Reflowable falls through to
  the existing CFI->percentage path, unchanged.

templates/reader.templ

@@ -19,6 +19,14 @@ func readerInitExpr(metadata ReaderMetadata, progress ReadingProgress) string {
 	if progress.EpubCfi != "" {
 		config["savedCfi"] = progress.EpubCfi
 	}
+	// Fixed-layout & comic formats: the page index is the canonical, exact
+	// locator (pages are fixed images). Pass it so the reader restores by page.
+	if (metadata.FormatGroup == "fixed_layout" || metadata.FormatGroup == "comic_archive") && progress.CurrentPage > 0 {
+		config["savedPage"] = progress.CurrentPage
+		if progress.TotalPages > 0 {
+			config["savedTotalPages"] = progress.TotalPages
+		}
+	}
 	jsonBytes, _ := json.Marshal(config)
 	return fmt.Sprintf("initReader(%s)", string(jsonBytes))
 }